1. Field of the Invention
The present invention relates to an optical projection system suitable for projecting an object plane, i.e., an original image, on an image plane in optical equipment such as electrophotographic copying machines and facsimile equipment. More particularly, the present invention concerns an optical projection system suitable for projecting an object plane on an image plane at various magnifications, including reduced and enlarged scales, using a so-called compound eye system in which a plurality of lens elements comprised of converging optical transmitters, microlenses, or the like are disposed in rows.
2. Related Background Art
Hitherto, in optical equipment such as electrophotographic copying machines and facsimile equipment, an object plane is projected on an image plane at a predetermined magnification by using a compound eye system.
The use of such a compound eye system has advantages in that, since the shooting field angle of a lens element may be narrow for projecting a given image plane, favorable optical performance can readily be obtained, and in that the overall optical length (i.e., the distance from the object plane to the image plane) can be made short, with the result that the overall apparatus can readily be made compact. As for methods of shooting by using this compound eye system, the following two methods are employed: one using an erect real image system for projecting an object plane on an image plane as an erect real image, and the other using an inverted real image system for projecting the same on an image plane as an inverted real image.
FIG. 1 is a schematic diagram of a compound eye system of the erect equal (one to one) magnification type, which has been proposed in, for instance, Japanese Patent Laid-Open Publication No. 83001/1980. In the drawing, reference numeral 20 denotes a compound eye system comprised of a plurality of erect real image systems 21 each constituted by a converging optical transmitter, microlenses, or the like. The given scope of an object plane 1 is projected on an image plane 2 as erect equal-magnification images by means of the individual erect real image systems 21, and an integrated image is formed by making the erect equal-magnification images overlap each other. As a result, a large object plane which cannot be covered by an independent erect real image system can be projected on the image plane. The compound eye system shown in FIG. 1 employs an equal shooting magnification, so that the compound eye system is arranged in such a manner that the optical axes of the respective erect real image systems 21 become parallel, and that respective beams of light on the optical axes perpendicularly intersect the object plane 1 and the image plane 2. This arrangement makes it possible to form an integrated image by overlapping the images projected on the image plane by means of the respective erect real image systems 21, i.e., the so-called multiple images.
However, in FIG. 1, if the object distance is varied to convert the optical projection system into a reduced or enlarged system, the multiple images formed by the erect real image systems do not consistently overlap on the image plane and are offset from each other due to the discrepancy between the predetermined magnification and the magnification of a portion between optical axes on the image plane, thereby resulting in a so-called "mismatching of images". This phenomenon of mismatching of images causes a substantial decline in the optical performance of a projected image.
To cope with this problem, a method of compensating for the mismatching of multiple images which occurs when reduced projection or enlarged projection is carried out using a compound eye system has been proposed in, for example, Japanese Patent Laid-Open Publication No. 16415/1982. In this publication, a plurality of erect real image systems 31 constituting a compound eye system 30 are arranged in such a manner that their optical axes are inclined gradually with respect to the optical axis 311 of the central erect real image system 310, thereby compensating for the mismatching of multiple images. With this compound eye system, however, the optical performance and the optical axis length (the optical length of the optical axis from the object plane to the image plane) vary depending on each erect real image system. In addition, with this compound eye system, the beams of light on the optical axes of the erect real image systems for projecting the periphery of the object plane deviate substantially from the perpendicular when such beams intersect the object plane and the image plane. For this reason, in the case of an erect real image system 41 which is inclined at a considerable angle, as shown in FIG. 3, the object plane whose projection magnification is equal, is inclined from the normal object plane 1 and becomes like the object plane 42.
Meanwhile, the image plane whose projection magnification is equal similarly, becomes inclined from the normal image plane 2 and becomes like an image plane 43. Consequently, there is a so-called "variation in magnification", in which the magnification of image formation differs partially within field of view by an amount corresponding to the difference in length between optical paths 141 and 142, as shown in FIG. 3.
Thus, with a conventional optical projection system employing a compound eye system, this variation in magnification occurs even if compensation is made for the mismatching of multiple images, so that it is difficult to obtain a projected image having high optical performance in effecting projection with magnifications other than the equal magnification.
Furthermore, in the same publication, an attempt is made to alleviate the variation in magnification. by off-centering an incident end surface or emergent end surface of each erect real image system or by adding a refracting power thereto. In this arrangement, however, the overall optical projection system becomes complex. In theory, the optical axis of each of the erect real image systems cannot be perpendicular to the object plane and the image plane. Therefore, there are limitations to such compensation, so that it is difficult to substantially overcome the variation in magnification by such a means.
In addition, other optical projection systems in which the mismatching of multiple images at the time when reduced projection or enlarged projection is effected is corrected using a compound eye system constituted by a plurality of erect real image systems have been proposed in, for example, Japanese Patent Laid-Open Publication Nos. 45420/1984 and 216115/1984.
As shown in FIG. 4, the Japanese Patent Laid-Open Publication No. 45420/1984 proposes an optical projection system in which the mismatching of multiple images is compensated for by an arrangement in which luminous flux deflecting members 52, 53 constituted by Fresnel lenses or the like having different deflection angles for respective erect real image systems are disposed on at least one of the object plane 1 side and image plane 2 side of a compound eye system 50 constituted by a plurality of erect real image systems 51.
Meanwhile, as shown in FIG. 5, the Japanese Patent Laid-Open Publication No. 216115/1984 proposes an optical projection system in which the mismatching of multiple images is compensated for by an arrangement in which a plurality of spherical lenses 62, 63 are disposed on at least one side of the object plane 1 side and image plane 2 side of a compound eye system 60 constituted by a plurality of erect real image systems 61.
However, in both of the optical projection systems proposed in the aforementioned two publications, the optical axis lengths of the respective erect real image systems differ, and the optical axes of the erect real image systems for projecting the periphery of the object plane are substantially inclined relative to the object plane and the image plane. Consequently, although it is possible to compensate for the mismatching of multiple images, as described above, the variation in magnification occurs, causing a substantial decline in the optical performance of a projected image.
In addition, in the other method of an optical projection system employing an inverted real image system, the "mismatching of images" and the "deviation in magnification" also occur due to similar causes. However, such an inverted real image system is capable of shortening the overall optical length since image formation can be effected once as compared with two-times image formation necessary for an erect real image system. Therefore, the inverted real image system has an advantageous feature in that the overall optical systems can be simplified, so that the overall apparatus can be made compact.
As such, the inverted real image system is employed in many types of electrophotographic copying machines. With the inverted real image system, however, the object image rotates 180.degree. on the image plane, it is necessary to overlap on the image plane object images formed by the respective inverted real image systems, by using reflecting mirrors or the like to maintain a give relationship among the object images.
For instance, as shown in FIG. 6A, when projecting an object 23 placed on the object plane 1 on the image plane 2 using two inverted real image systems 221, 222, if a given scope of the object plane 1 is merely projected on the image plane by the inverted real image systems 221, 222, a so-called "image mismatching by rotation" of 180.degree. occurs in which an image 24-1 formed by the inverted real image system 221 and an image 24-2 formed by the inverted real image system 222 are offset from each other, as shown in the drawing.
As a result, it becomes impossible to accurately project an object image. Therefore, as shown in FIG. 6B, an arrangement has hitherto been made so that respective object images 26-1, 26-2 of an object image 26 are projected accurately on the image plane 2 using a plurality of image rotating means 25 such as reflecting mirrors.
However, in order to arrange objects 23-1, 23-2 on the image plane 2 such as to form the object images 26-1, 26-2, it is necessary to dispose the plurality of reflecting mirrors in a complex manner, with the result that the overall apparatus has tended to become large in scale.